The term “ultrasound” typically applies to acoustic energy with a frequency above human hearing (20,000 hertz or 20 kilohertz). When used in medical applications, ultrasound is typically between 1 and 30 MHz for imaging and flow measurements and between 0.05 and 1.00 MHz for therapy. The application of ultrasound in medicine began in the 1950s. It was first introduced in the field of obstetrics. Obstetric sonography is the use of medical ultrasonography in pregnancy, in which sound waves are used to create real-time visual images of the developing embryo or fetus in its mother's uterus. The procedure is a standard part of prenatal care in many countries, as it can provide a great deal of information about the health of the mother, the timing and progress of the pregnancy, and the health and development of the embryo or fetus. After that, the use of ultrasound propagated to nearly all fields of medicine including abdominal diagnostics, cardiology, urology, cerebrovascular, ophthalmology, orthopedics, breast examination, and pediatrics. Ultrasound has been proven to provide fast, accurate and safe patient imaging for an expanding array of diagnostic and therapeutic applications with ongoing technologic improvements and the growing recognition of harmful radiation from other imaging modalities.
Sonographers typically use a hand-held probe (called a transducer) that is placed directly on and moved over the patient. With the use of the probe, the sonographer is able to visualize body structures under the skin including tendons, muscles, joints, nerves, vessels and internal organs for possible pathology or lesions. Current probes utilize reflection technology. The probe transmits high-frequency ultrasound sound pulses into the body. The pulses are produced by a piezoelectric transducer within the probe. Strong, short electrical pulses from the ultrasound machine cause the crystals to change shape rapidly. The rapid shape changes, or vibrations, of the crystals produce sound waves that travel outward. The ultrasound wave travels into the body until it encounters a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). Some of the ultrasound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected. The reflected waves are picked up and interpreted by the probe to produce a real-time two-dimensional representation on a monitor. Interpretation through the probe occurs when the reflected sound or pressure waves hit the piezoelectric crystals which causes them to emit electrical currents. Thus, the same crystals can be used to send and receive sound waves. The probe also has a sound absorbing substance to eliminate back reflections from the probe itself. The electric currents generated by the reflected waves are relayed to the ultrasound machine. The ultrasound machine is able to calculate the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (5,005 ft/s or 1,540 m/s) and the time of each echo's return (usually on the order of millionths of a second). The ultrasound machine then displays the distances and intensities of the echoes on the screen, forming a two-dimensional image. 3D images can be generated by acquiring a series of adjacent 2D images by simply moving or tilting the probe on the patient.
In order for the maximal transmission of energy from one medium to another (i.e, from the probe through the skin), the impedance of the two media should be nearly the same. Clearly, in the case of ultrasound waves passing from the probe to the tissues, this cannot be readily achieved. The greater the difference in impedance at a boundary, the greater the reflection that will occur, and therefore, the smaller the amount of energy that will be transferred. With decreased sound waves transferred, there is less energy to be reflected and interpreted by the probe. The difference in impedance is greatest for the probe/air interface which is the first one that the ultrasound has to overcome in order to reach the body. Therefore, maintaining constant and optimal contact between the probe and skin is important for the utilization of ultrasound technology. To minimize this difference, a suitable coupling medium is typically used. The coupling media used in this context includes various oils, creams and gels. The most popular is gel which is applied to the probe head and/or the body of the patient. The ultrasound coupling gel displaces air and fills contours between the piezoelectric eye, or transducer, of an ultrasound instrument (such as a probe or scanhead), which converts energy between electrical and acoustic, and the body into which the sound is being directed. Examples of ultrasound probes or scanheads can be found in U.S. Pat. No. 5,482,047 to Nordgren et al. or U.S. Pat. No. 5,207,225 to Oaks et al. This gel or fluid material, by nature of its physical and acoustic properties, serves as an ultrasound acoustic coupler between the ultrasound transducer and tissue, thereby acoustically joining the two, so that the ultrasound based information developed can freely pass back and forth between the body and the transducer.
Because of the coupling effect, this media is commonly referred to as an ultrasound couplant, ultrasound gel, scanning gel, ultrasound transmission media or acoustic transmission media. Many fluids and water-based gels have been used as ultrasound couplants over the years. Early use of mineral oil was replaced by gels whose thickness was provided from a polymer group consisting of a copolymer of methyl vinyl ether, maleic anhydride, carboxy polymethylene polymer and mixtures thereof, or from a mixture of carboxy polymethylene polymer neutralized with an alkaline agent as a primary thickener together with hydroxy alkyl cellulose as an auxiliary thickener and a polyalkylene glycol such as propylene glycol as a humectant, as described in U.S. Pat. No. 4,002,221 to Buchalter and U.S. Pat. No. 4,459,854 to Richardson et al.
Fluids and gels commonly used as ultrasound couplants have several fundamental disadvantages, some of which are described herein. To begin, patients often find the fluid or gel to be cold, sticky and messy. The fluids or gels are difficult to contain on, and remove from, the patient during and after the ultrasound procedure. Further, commercially available oils and water based gels often introduce problems to the electronics by their chemically degrading nature. They may react with the adhesives, elastomers, and epoxies used in the construction of medical ultrasound transducers, thus appreciably degrading performance and shortening their service life. With therapeutic interventions such as needle biopsies or nerve blocks, the gels may be introduced into the body which introduces additional infectious or inflammatory risks to the patient as described in further detail below.
In addition, fluids and gels offer no microbial barrier between the patient and the probe transducer; thus, latex rubber or synthetic elastomer probe covers must be applied over the probe transducer, to prevent transmission of microorganisms to the patient. Often, two layers of couplant, one inside and one outside the probe cover, are required to provide ultrasound acoustic coupling between transducer and the patient. This potential infection concern is readily apparent when the transducer is used for imaging during needle biopsy or aspiration, or inside the body during surgery in direct organ, tissue and blood contact. Of growing importance is the protection from infection by skin transmission to patients who are immune compromised by disease, organ replacement, immune system modification, chemotherapy or radiation treatments. Ultrasonic gel has been observed to have many microbial and clinical challenges as evidenced by many clinical papers. In addition, the US Food and Drug Administration has issued several warnings about microbial contaminants related to the ultrasound gel. It has also resulted in closing of a company. Furthermore, there are several papers published describing the impact of microbial issues related to the ultrasound gel.
Fluids or thickened water-based gels typically used in medical ultrasound, similarly described as in U.S. Pat. No. 4,002,221, are comprised of chemical compounds such as acrylic polymers, carboxy alkyl cellulose, hydroxyethylcellulose, carboxy polymethylene, organic acids, alkali metal salts, parabens and other germicidal and fungicidal agents, and surfactants. Such chemicals are not approved or suitable for use in applications where they may be carried into the body, such as during biopsy, intra-operative procedures, or when the transducer is placed inside a body orifice. In instances where sterile latex rubber or synthetic covers containing thickened ultrasound coupling gels are used in surgery, tearing, cutting, or rupture of the cover results in the tissue incompatible ultrasound coupling gel spilling into the body cavity. During ultrasound guided needle biopsy, aspiration, intracavity and intraoperative procedures, sterile covers produced from latex, polyurethane, polypropylene and other polymers, such as described in U.S. Pat. No. 4,593,699 to Poncy et al., U.S. Pat. No. 5,259,383 to Holstein et al. and U.S. Pat. No. 5,676,159 to Navis, are used with such tissue incompatible gel chemicals. A puncturing needle can carry such chemicals into the body, such as into the breast or into amniotic fluid, since gels are present on the skin of the patient at the point of needle insertion, as well as between the transducer and the probe cover to accomplish ultrasound acoustic coupling. Thus, as a puncturing needle passes through the gel on the skin of the patient, minute quantities of the gel may be carried into the underlying tissue and the body cavity thereby introducing a likely tissue-incompatible substance into the patient. It is apparent that this gel may also harbor bacterial organisms from manufacturing or transfer from local sources during clinical use which can then also be transferred into the body.
In addition, many practitioners also have difficulty with consistency of application. It is difficult for many practitioners to apply enough gel within a probe cover to prevent air pockets, to remain thick enough and evenly applied between the probe and cover throughout the examination without ‘spilling around’ edges of the probe, and without causing uneven ‘wrinkles’ or curvatures of the cover which causes air pockets outside the probe cover. This increases procedure time and results in suboptimal visualization of underlying structures and interferes with the quality of examination or procedure.
Therefore, improved methods and devices are desired to reliably and safely provide maximal transmission of acoustic energy during ultrasound imaging while reducing fundamental disadvantages associated with the conventional use of ultrasound couplants. At least some of these objectives will be met by the present invention.